Electric railway vehicles such as locomotives or rail coaches powered by an alternating-current (AC) supply line use a traction transformer and an AC/DC converter for converting the high voltage (15 kV or 25 kV) of the supply line to a direct-current (DC) link voltage of a few kV and to ensure a galvanic separation between the high voltage and the traction circuits. A DC link or bus at the DC link voltage feeds the drive or motor converters for traction or propulsion of the vehicle, as well as auxiliary converters for auxiliary energy supply.
In modern railway vehicle concepts, the traction transformer is usually positioned outside the main casing of the vehicle, i.e. under floor or on the rooftop. In these places however, a conventional transformer with a nominal frequency of 16.7 Hz or 50 Hz causes integration problems due to its high weight and large volume. Alternative power supply systems therefore aim at replacing the aforementioned conventional transformer by additional power electronic converters based on semiconductor technology in combination with a smaller and lighter transformer operating at a higher frequency. At the expense of switching losses in the semiconductor devices, the mass and volume of the transformer as well as the total, i.e. copper and magnetic, losses in the transformer can thus be reduced, resulting in a more efficient use of the electrical power from the supply line.
In the patent application EP-A 1 226 994, a medium frequency power supply system for rail vehicles is presented, including a classical converter topology for the bidirectional conversion of a high input AC voltage to a DC output voltage. The system comprises a primary converter composed of at least three cascaded converter modules or sections electrically connected in series, one single common transformer and a single secondary converter. Each cascade module in turn is formed by a four-quadrant converter, a 3.6 kV DC intermediate stage and a resonant converter. The secondary or output converter is a resonant switched four-quadrant converter feeding the vehicle's 1.65 kV DC link. All switching elements are advanced 6.5 kV Insulated Gate Bipolar Transistors (IGBT) with an adapted gate driver technology. Instead of passing through a DC intermediate energy storage stage, conversion from the supply line frequency to the transformer frequency can also be accomplished directly by a direct AC frequency converter, also known as a cycloconverter.
In medium voltage multilevel converters, the semiconductor devices of the different converter levels or stages are at different absolute electrical potential levels and have to be insulated against each other and against ground potential. Likewise, the conventional gate-drive units connected to the gates of and controlling the semiconductor devices via suitable low-voltage control signals as well as the power sources providing power to the gate-drive units have to be galvanically separated. Typically, opto-couplers are then used to translate the activation command from a ground referenced control circuit up to the gate-driver at floating medium voltage level.
According to the publication WO 9406209, a gate-drive circuit comprises an insulation boundary including a 2 MHz push-pull converter, a transformer and a rectifier operatively coupled to a secondary side of the transformer. The power required to switch the IGBT devices is supplied by a ground referenced 24 VAC or 30 VDC source, sent through the converter and provided to the gate-drive unit by the rectifier. As the gate-drive circuit including the insulation boundary is part of one single integrated gate-drive unit, insulation problems due to space restrictions or dielectric requirements prohibits a straightforward application of this gate-drive unit to higher voltage levels beyond approximately 10 kV RMS. On the other hand, UltraVolt Inc. (www.ultravolt.com) offers power supplies for high voltages ranging up to 40 kV and for power levels ranging from 4 to 250 Watts. However, in this case different voltage levels would each require their own dedicated power supply.